1. Field of the Invention
The invention relates to holographic storage. More particularly, the invention relates to an apparatus and method for producing encoded optical beams within holographic memory systems.
2. Description of the Related Art
Holographic memory systems involve the three-dimensional storage of holographic representations (i.e., holograms) of data elements as a pattern of varying refractive index and/or absorption imprinted in the volume of a storage medium such as a crystal of lithium niobate. Holographic memory systems are characterized by their high density storage potential and the potential speed with which the stored data is randomly accessed and transferred.
In general, holographic storage memory systems operate by combining a data-encoded object beam with a reference beam to create an interference pattern throughout a photosensitive storage medium such as a holographic memory cell (HMC). The interference pattern induces material alterations in the storage medium that generate a hologram.
The object beam typically is formed by transmitting laser light through a spatial light modulator (SLM), e.g., a liquid crystal display (LCD) screen or a digital micro-mirror device (DMD). The SLM typically contains a pattern of clear and opaque (pixel) regions corresponding to a two-dimensional depiction of the digital data to be stored. The laser light signal emanating from the SLM is then relayed, e.g., by lenses, to create the object beam.
The formation of the hologram in the holographic memory cell (HMC) or other suitable storage medium is a function of the relative amplitudes and polarization states of, and phase differences between, the object beam and the reference beam. It is also highly dependent on the wavelengths and angles at which the object beam and the reference beam are projected into the storage medium. In this manner, it is possible to encode data, e.g., by phase, angle or wavelength, prior to storage of the encoded data within the storage medium.
It is possible to further encode the object beam generated by the SLM by using, e.g., phase plates or lenses. These additional encoding techniques work to improve object beam intensity uniformity at the recording media, which typically results in improved system signal-to-noise ratio and storage density.
Holographically-stored data is reconstructed by projecting a reference beam similar to the reference beam used in storing the data into the HMC at the same angle, wavelength, phase and position used to produce the hologram. The hologram and the reference beam interact to reconstruct the stored object beam. The reconstructed object beam then is detected, e.g., using a photodetector array. Once detected, the recovered data is, e.g., post processed for delivery to output devices.
During phase-encoding, the data emanating from the spatial light modulator (SLM) is directed through a phase mask (such as lenses, lens arrays or random phase masks) prior to storage within a storage medium. When the spatial light modulator (SLM) is illuminated on-axis (i.e., when the illumination beam is perpendicular to the plane of the SLM), the phase mask typically is placed in contact with the SLM. However, when the spatial light modulator (SLM) is operating in an off-axis mode, the phase mask typically is placed in the image plane of the SLM following the SLM to ensure proper operation of the phase mask. However, such an arrangement typically requires additional optics between the SLM and the phase mask.
Therefore, it is desirable to provide a relatively inexpensive, simple and reproducibly consistent replacement for one or more of the elements that typically are required of phase encoding optical systems with off-axis spatial slight modulators (SLMs).